Measurement and correction of multiphase clock duty cycle and skew

ABSTRACT

Methods and systems are described for generating, at a plurality of delay stages of a local oscillator, a plurality of phases of a local oscillator signal, generating a loop error signal based on a comparison of one or more phases of the local oscillator signal to one or more phases of a received reference clock, generating a plurality of phase-specific quadrature error signals, each phase-specific quadrature error signal associated with a respective phase of the plurality of phases of the local oscillator signal, each phase-specific quadrature error signal based on a comparison of the respective phase to two or more other phases of the local oscillator signal, and adjusting each delay stage according to a corresponding phase-specific quadrature error signal of the plurality of phase-specific quadrature error signals and the loop error signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/854,681, filed Apr. 21, 2020, naming Milad Ataei Ashtiani, entitled “Measurement and Correction of Multiphase Clock Duty Cycle and Skew”, which is a continuation of U.S. application Ser. No. 16/378,476, filed Apr. 8, 2019, now naming Milad Ataei Ashtiani, entitled “Measurement and Correction of Multiphase Clock Duty Cycle and Skew”, which is hereby incorporated by reference in its entirety for all purposes.

REFERENCES

The following prior applications are herein incorporated by reference in their entirety for all purposes:

U.S. Patent Publication 2018/0115410A1 of application Ser. No. 15/791,373, filed Oct. 23, 2017, naming Armin Tajalli and Amin Shokrollahi, entitled “Quadrature and Duty Cycle Correction in Matrix Phase Lock Loop”, hereinafter referred to as [Tajalli I].

FIELD OF THE INVENTION

The present invention relates to communications systems circuits generally, and more particularly to obtaining stable, correctly phased receiver clock signals from a high-speed multi-wire interface used for chip-to-chip communication.

BACKGROUND

In modern digital systems, digital information has to be processed in a reliable and efficient way. In this context, digital information is to be understood as information available in discrete, i.e., discontinuous values. Bits, collection of bits, but also numbers from a finite set can be used to represent digital information.

In most chip-to-chip, or device-to-device communication systems, communication takes place over a plurality of wires to increase the aggregate bandwidth. A single or pair of these wires may be referred to as a channel or link and multiple channels create a communication bus between the electronic components. At the physical circuitry level, in chip-to-chip communication systems, buses are typically made of electrical conductors in the package between chips and motherboards, on printed circuit boards (“PCBs”) boards or in cables and connectors between PCBs. In high frequency applications, microstrip or stripline PCB traces may be used.

Common methods for transmitting signals over bus wires include single-ended and differential signaling methods. In applications requiring high speed communications, those methods can be further optimized in terms of power consumption and pin-efficiency, especially in high-speed communications. More recently, vector signaling methods have been proposed to further optimize the trade-offs between power consumption, pin efficiency and noise robustness of chip-to-chip communication systems. In those vector signaling systems, digital information at the transmitter is transformed into a different representation space in the form of a vector codeword that is chosen in order to optimize the power consumption, pin-efficiency and speed trade-offs based on the transmission channel properties and communication system design constraints. Herein, this process is referred to as “encoding”. The encoded codeword is communicated as a group of signals from the transmitter to one or more receivers. At a receiver, the received signals corresponding to the codeword are transformed back into the original digital information representation space. Herein, this process is referred to as “decoding”.

Regardless of the encoding method used, the received signals presented to the receiving device must be sampled (or their signal value otherwise recorded) at intervals best representing the original transmitted values, regardless of transmission channel delays, interference, and noise. This Clock and Data Recovery (CDR) not only must determine the appropriate sample timing, but must continue to do so continuously, providing dynamic compensation for varying signal propagation conditions.

Many known CDR systems utilize a Phase-Locked Loop (PLL) or Delay-Locked Loop (DLL) to synthesize a local receive clock having an appropriate frequency and phase for accurate receive data sampling. In advanced embodiments, multiple local clocks with particular phase relationships may be generated, as one example to permit overlapping or parallel processing of received information by multiple instances of the receiver embodiment.

BRIEF DESCRIPTION

Data receivers require accurately adjusted local clocks to enable accurate signal detection, and advanced receiver designs may require generation of multiple clock phases, collectively having particular relationships with the received data signals, and fixed phase relationships among each other.

A common receiver clock subsystem utilizes a phase-locked loop (PLL) to produce a local clock having the desired frequency and phase relationship with a reference signal, generally obtained with or derived from the received data. Within the PLL, a voltage-controlled oscillator based on a ring-connected sequence of active elements conveniently produces multiple clock phases in a fixed relationship. However, variations among the ring's active elements can also induce periodic clock variations and thus result in undesirable duty cycle variations as well as skew between output clock phases.

A configurable clock buffer chain is described, allowing adjustment of clock duty cycle and overall delay by internally modifying the rise and fall time of signals propagating between buffer stages. These buffers are combined with a measurement subsystem capable of directly measuring clock duty cycle and inter-phase skew, to provide clean, accurately timed multiphase clock signals to the data receiver.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a block diagram of one clock generator embodiment, producing multiple output clocks in a desired relationship.

FIG. 2A is a block diagram of an embodiment measuring the duty cycle and phase relationship of two clock signals.

FIG. 2B is a block diagram of the control logic of FIG. 2A, in accordance with some embodiments.

FIG. 3 is a schematic diagram of a clock buffer chain with configurable delay characteristics.

FIG. 4 illustrates a technique for adjusting a duty cycle of a clock signal and corresponding waveforms, in accordance with some embodiments.

FIG. 5 illustrates a technique for adjusting rising-edge to rising-edge delays between two clock signals and corresponding waveforms, in accordance with some embodiments.

FIG. 6 is a block diagram of a single-ended to differential divider, in accordance with some embodiments.

FIG. 7 illustrates waveforms of various signals generated in a rising-edge to rising-edge delay correction process, in accordance with some embodiments.

FIG. 8 is a flowchart of a method 800, in accordance with some embodiments.

FIG. 9 is a block diagram of a control signal generator and synchronizer, in accordance with some embodiments.

FIG. 10 is a waveform illustrating incremental corrections made over a 1 microsecond time period, in accordance with some embodiments.

DETAILED DESCRIPTION

To reliably detect the data values transmitted over a communications system, a receiver must accurately measure the received signal value amplitudes at carefully selected times. Various methods are known to facilitate such receive measurements, including reception of one or more dedicated clock signals associated with the transmitted data stream, extraction of clock signals embedded within the transmitted data stream, and synthesis of a local receive clock from known attributes of the communicated data stream. In general, the receiver embodiments of such timing methods are described as Clock-Data Recovery (CDR) or alternatively as performing Clock-Data Alignment (CDA). These timing methods are often based on Phase-Lock Loop (PLL) or Delay-Locked Loop (DLL) synthesis of a local receive clock having the desired frequency and phase characteristics.

In both PLL and DLL embodiments, a Phase Detector compares the relative phase (and in some variations, the relative frequency) of a received reference signal and a local clock signal to produce an error signal, which is subsequently used to correct the phase and/or frequency of the local clock source and thus minimize the error. As this feedback loop behavior will lead to a given PLL embodiment producing a fixed phase relationship (as examples, 0 degrees or 90 degrees of phase offset) between the reference signal and the local clock, an additional fixed or variable phase adjustment is often introduced to permit the phase offset to be set to a different desired value (as one example, 45 degrees of phase offset) to facilitate receiver data detection.

Advanced receiver embodiments may require the generation of two or more local clocks having particular phase relationships. As one example, a so-called “four phase” embodiment incorporates four instances of detection apparatus configured to operate on consecutive unit intervals of the received signal, with the resulting parallelism providing extended detection time. In such a system, four phases of local clock signals may be required having a fixed frequency and phase relationship to the reference signal, and also having fixed relationships to each other.

PLL Overview

Phase Locked Loops are well represented in the literature. A typical PLL is composed of a phase detector that compares an external reference signal to an internal clock signal, a low pass filter that smooths the resulting error value to produce a clock control signal, and a variable frequency clock source (typically, a Voltage Controlled Oscillator or VCO) controlled by the smoothed error value, producing the internal clock signal presented to the phase detector.

In an alternative embodiment, the variable frequency clock source is replaced by a variable delay element, its (optionally multiple tapped) outputs thus representing one or more successive time-delayed versions of the original input signal rather than successive cycles of an oscillator to be phase compared to the reference input signal. For the purposes of this document, such Delay Locked Loops (DLL) are considered functionally equivalent to a PLL in such an application, and the tapped variable delay element of a DLL functionally equivalent to the ring of delay elements in a PLL ring-oscillator VCO.

In one embodiment, a ring oscillator composed of a sequence of identical gates in a closed loop is used as the internal Voltage Controlled Oscillator (VCO) timing source for the PLL. The VCO frequency is varied by analog adjustment of at least one of gate propagation delay, inter-gate rise and fall time, and gate switching threshold within the ring oscillator. As examples, the supply voltage or current provided to the ring oscillator elements may be adjusted to modify internal node switching time and thus the resulting oscillation frequency. Outputs taken at equal intervals (i.e. separated by equal numbers of ring oscillator gates) along the sequence of gates comprising the ring oscillator can provide multi-phase clocks having a fixed phase relationship. Such ring oscillators are well represented in the art, typically comprised of three to eight or more elements typically embodied as digital inverters, with both single-ended and differential signal variations described in the literature.

The example embodiment illustrated in FIG. 1 incorporates ring oscillator 110 to generate clock signals VCO Phase 1 and VCO Phase 2. In this example, 110 utilizes three differential delay elements (shown here as differential inverters) connected in a ring, with the two VCO clock outputs having a fixed 180 degree phase relationship with each other.

It is known that periodic variations in edge timing of a receiver's local clock signals can lead to degraded signal detection quality, thus it is extremely desirable to minimize these effects. In the example of FIG. 1, Clock Phase 1 and Clock Phase 2 are ideally perfect square waves of constant frequency, with exact 50% duty cycles and zero differential phase error or “skew.” In practice, however, it is recognized that unavoidable variations among the ring oscillator elements can result in asymmetric output waveforms and periodic timing variations.

[Tajalli 1] describes a ring oscillator embodiment in which multiple ring oscillator output phases are compared against each other using a matrix phase detector. The resulting differential phase error information is used to incrementally adjust the delay of each ring oscillator element, above and beyond the overall frequency and phase error correction applied to the ring oscillator as a whole by the primary PLL phase detector.

One embodiment of the system shown in FIG. 1 includes a ring oscillator operating at frequencies near the practical limits of the integrated circuit process used. Thus, a minimal three-stage ring was designed, with an extremely tight physical layout to minimize parasitic node capacitance. In this design, the extra metallization lines needed to bring in individual delay controls for each stage, and the additional loading introduced by bringing out individual ring phases for matrix comparison, introduced unacceptable constraints on the desired maximum VCO operating frequency.

Instead of directly manipulating ring oscillator elements, the system of FIG. 1 processes selected clock outputs Clk₁ and Clk₂ selected via clock selection circuit 115 using configurable buffer chains 120 and 160, allowing each output clock Clock Phase 1 and Clock Phase 2 to be brought to the desired 50% duty cycle and zero differential phase before use by the receiver system. In some embodiments, the sources used to generate Clk₁ and Clk₂ may be selected via the clock selection circuit 115, shown in FIG. 1 as a differential multiplexer. Such a clock selection circuit may have inputs from the VCO 110 and inputs provided via a phase interpolator (PI) 117 that may be operating e.g., on outputs of the VCO. Thus, the duty cycle and delay correction circuit may be shared between the “main” clocks and the “PI” clocks. For descriptive simplicity, Clk₁ and Clk₂ are assumed to be full-swing CMOS clock signals with a 180-degree phase offset with no limitation implied.

Measurement subsystem 200 observes the resulting outputs Clock Phase 1 and Clock Phase 2, measuring individual clock duty cycles Clk1_duty and Clk2_duty respectively, and the differential clock offset between Clk₁ and Clk₂, denoted Clk_skew. In some embodiments, the delay correction Clk_skew may include a rising-edge to rising-edge (RE-to-RE) component and a falling-edge to falling-edge (FE-to-FE) component. The control logic 240 provides multi-bit control signals for adjusting stages 121, 122, and 123 in configurable clock buffer chains 120 and 160 to maintain the desired result at their outputs. FIG. 2B is a block diagram of an exemplary control logic 240, in accordance with some embodiments.

As shown, control logic 240 includes a selection circuit 293, shown in FIG. 2B as a multiplexer. The multiplexer may be a differential multiplexer configured to receive differential inputs corresponding to (i) the duty cycle corrections for the first and second clock signals Clk1_duty and Clk2_duty, respectively, and delay corrections generated from the inter-phase comparison of the edge-triggered half-rate clocks, shown as FE-to-FE_delay and RE-to RE_delay. The selection circuit 239 may be configured to incrementally select the inputs, and to provide the selected input to a shared low-pass filter. The filtered result may be provided to control signal generator 295, which generates the multi-bit control signals EnP<13:7>, EnN<13:7>, EnPb<6:0> and EnNb<6:0>. The control signal generator 295 may be configured to synchronize the multi-bit control signals according to a digital flag syn_dig and a two UI clock Clk_2 ui. FIG. 9 is a block diagram of an exemplary control signal generator 295, in accordance with some embodiments. FIG. 10 illustrates waveforms of an incremental update process, in accordance with some embodiments.

FIG. 2A shows additional details of measurement system 200. If inputs Clk₁ and Clk₂ are full-swing 50% duty cycle CMOS signals, their time-averaged DC/common mode level will be Vdd/2, or one-half of the total signal excursion. For the first clock signal Clk₁, Low-pass filter 210 performs such a time-averaging operation, with analog result 211 representing the average DC level of the first clock signal. Comparator 220 compares 211 to a fixed DC reference of Vdd/2, with the duty cycle correction Clk1_duty indicating whether the duty cycle of the first clock signals Clk₁ is greater than, or less than the desired 50% value. Similarly, Low-pass Filter 250 generates the analog result 251 representing the average DC level of the second clock signal and Comparator 260 compares 251 to the reference voltage Vdd/2 to produce duty cycle correction Clk2_duty. A 50% duty cycle approach should not be considered limiting, and another appropriate fixed DC reference may be set for inputs having a different desired duty cycle or different voltage swing.

Digital divide-by-2 flip-flops 230 and 270 produce rising-edge (RE) and/or falling-edge (FE) triggered half-rate square wave signals 231 and 271 from rising/falling edges respectively of Clk₁ and Clk₂ which are then compared by phase detector 280, shown here as a simple XOR gate. In some embodiments, both RE and FE triggered half-rate clocks are generated, while alternative embodiments may utilize a single edge-triggered half-rate clock to reduce convergence time for duty cycle and clock skew. FIG. 7 illustrates various waveforms generated for duty cycle corrections between rising edges of Clk₁ and Clk₂, in accordance with some embodiments. As shown, Clk₁ and Clk₂ have a rising-edge to rising-edge delay that is greater and a single unit interval (UI). Edge-triggered half-rate clocks are generated for each of Clk₁ and Clk₂, shown as Clk₁/2 and Clk₂/2, respectively. An inter-phase comparison is formed by taking a logical XOR of the edge-triggered half-rate clocks. As shown in FIG. 7, the inter-phase comparison has a duty cycle >50%, indicative of the >1 UI delay between the rising edges in Clks 1 and 2. Such a delay correction may then be applied to elements controlling the rising edges of Clks 1 and 2, described in more detail below. As Clk₁ and Clk₂ in this example have a 180-degree phase differential, clocking the divide-by-2 flops with the same edge (e.g. rising clock edge) of both clocks will result in edge-triggered half-rate clock signals having a 90-degree phase differential, absent any skew in the original clock signals. Thus, delay correction 281 will ideally be a perfect square wave of 50% duty cycle with any skew between the input clocks presented as a duty cycle error in the delay correction which may be measured as previously described, using low-pass filter 285 and comparator 290 to produce delay correction Clk_skew. To ensure a valid result, dividers 230 and 270 may be initialized to a known state at startup (as one example, such that that first positive-going transition of 231 will precede the first positive-going transition of 271) to ensure the desired phase relationship between their outputs can be observed. FIG. 7 includes the resulting interphase comparison of Clk₁′/2 XOR'd with Clk₂/2 that may occur absent any start up initialization. As shown, the duty cycle of the resulting erroneous waveform is <50%. Again, no limitation is implied, as a different desired clock phase relationship may be targeted by appropriate initialization of the divider initial state, transitioning edge, and/or DC comparison value for the resulting low-pass-filtered result.

One embodiment of 200 minimizes measurement errors by implementing all signals and signal processing elements differentially, with identical loading on both signal paths in each differential pair. Thus, as examples, differential signal Clk1 passes through a differential R-C low pass filter 210 to differential comparator 220. FIG. 6 is a block diagram of one particular implementation of divider 230 operating on clock signal Clk1, in accordance with some embodiments. As shown, divider 230 includes a true single-phase clock (TSPC) divider 610 configured to generate singled-ended half-rate clock Clk₁/2 from single-ended clock signal Clk₁. Divider 230 further includes an inverter 615 configured to generate an inverted version Clk₁/2 of the single-ended half-rate clock signal Clk₁/2. The two single-ended half-rate clocks Clk₁/2 and Clk₁/2′ may then be retimed using a retiming circuit 620 according to the input clock signal Clk1. Divider 270 operating on Clk2 may include similar elements. The differential edge-triggered half-rate clocks 231 and 271 are inter-phase compared by differential phase comparator 280 before being low-pass filtered 285 and compared 290. In at least one embodiment, the corner frequency of each low pass filter is set approximately 1000 times lower than the clock frequency to provide a desired amount of DC averaging. In a further embodiment, two-stage low pass filters are used, a first filter having a corner frequency 10 to 100 times lower than the clock frequency, with its result directed to a second filter providing the remainder of the desired amount of DC averaging. In some embodiments, the second filter and measurement comparator are shared among multiple first filters and measurement points using an analog multiplexor, eliminating the need for multiple instances of the substantial second filter capacitance.

FIG. 3 details the internals of a configurable buffer chain, in accordance with some embodiments. In this embodiment, a series of digital inverters 310, 320, 340 amplify and buffer an input signal, resulting in an output suitable for driving a greater load and/or longer signal lines within an integrated circuit device. In some embodiments, the size of the transistors composing the inverter stages are scaled proportionately larger across the buffer chain, balancing increased drive capability with increased capacitive loading on the previous output stage. As one non-limiting example, the transistors within 320 may be twice the size and current-drive capability of those in 310, and those of 340 twice those of 320.

As illustrated in FIG. 3, the second stage of the buffer chain is composed of inverter 320, paralleled by seven instances of configurable augmentation inverter 330. As shown, each instance of 330 contains a pull-up transistor 331 and pull-down transistor 334 controlled by the same input signal as 320, and capable of driving the same output signal as 320. Enabling element 332, shown here as a switch, may be configured by control signal EnP to place pull-up transistor 331 into the active signal path, or to keep it isolated. In one representative embodiment, 332 and 333 are series MOS transistors. Similarly, control signal EnN controls enabling element 333, which may be configured to place pull-down transistor 334 into the active signal path, or keep it isolated. Enabling pull-up transistors 331 and pull-down transistors 334 may adjust the rising and falling edges of the eventual output signal Out of final output buffer stage 350. It should be noted that as there is a signal inversion at each stage of the illustrated buffer chain 310, 320, 330, the absolute clock edge associated with a “rising” and “falling” local signal edge will similarly reverse at each stage. For example, pull-up transistors 331 in stage 320 and pull-down transistors 354 in stage 340 may control the rising edge of the output signal Out (via enable signals EnP<13:7> and EnPb<6:0>, respectively) while pull-down transistors 334 in stage 320 and pull-up transistors 351 in stage 340 may control the falling edge of the output signal Out (via enable signals EnN<13:7> and EnNb<6:0>, respectively), or vice versa depending on the total number of buffer stages. Furthermore, pull-up transistors 331 and pull-down transistors 334 in stage 320 may include multiple transistors connected in parallel, increasing the effective size with respect to the transistors in stage 340.

FIGS. 4 and 5 illustrate exemplary behavior of instances 330, in accordance with some embodiments. As transistors 331 are enabled via EnP and transistors 334 are disabled via EnN, the duty cycle of the output signal Out increases due to the decrease in rise time of the rising edge and the increase in fall time of the signal on node 335, as shown in FIG. 4. Similarly, disabling transistors 331 and enabling transistors 334 may decrease the duty cycle of the output signal Out. FIG. 5 illustrates a mechanism for correcting rising-edge to rising-edge delay between Clks 1 and 2. As shown, disabling transistors 331 in the buffer generating Clk1 and enabling transistors 331 in the buffer generating Clk2 increases the rise time for Clk1 and decreases the rise time for Clk2, respectively. Such adjustments may create a one-unit interval alignment between the rising edges of clock signals Clk1 and Clk2.

When both EnP and EnN are enabled in a given parallel stage 330, stage 330 acts in parallel with 320 to provide an increased output drive current for both rising and falling edges of signal transitions on node 335, thus incrementally reducing the effective overall propagation delay of 310. Enabling only EnP provides increased drive (and thus, a faster transition time) only for rising transitions, and enabling only EnN provides increased drive (and thus, faster transition time) only for falling transitions. Other characteristics remaining constant, a faster rising transition time will incrementally increase the duration of active high levels of signal 321, and a faster falling transition time will incrementally increase the duration of active low levels of signal 312.

Seven parallel instances of 330 are shown, thus if control signals EnP<13:7> and EnN<13:7> are thermometer-coded, seven distinct amounts of augmentation may be configured for each of the rising and falling edge rates seen at node 321. Similarly, the seven instances of 350 can be configured to augment 340, using control signals EnP<6:0> and EnN<6:0>.

In one embodiment, transistors 331 and 334 within 330 are twice the size and current drive capability of the comparable transistors 351 and 354 in stage 340. As the transistors in 320 are themselves scaled to be twice the size of those in 340, one may observe that each step of augmentation provided by 330 may be 4× (assuming a 2× increase per stage) that provide by 350, thus EnP<13:7> and EnN<13:7> may be considered as “coarse” adjustment controls, and EnPb<6:0> and EnNb<6:0> as “fine” adjustment controls over the rising and falling edge characteristics of the signals being buffered by their respective stages.

As the “fine” and “coarse” control signals are thermometer-encoded where they are applied to each augmentation group, incremental control signal changes within each group are glitch-free. One particular embodiment insures that concurrent changes to both fine and coarse control signals are synchronized, by latching all control signals using a common clock. A further embodiment changes the amount of augmentation for a given edge transition only when the drivers for that edge are inactive. FIG. 9 illustrates such a mechanism for ensuring glitch-free incremental changes utilizing a digital flag syn_dig to latch the outputs of most significant bits gray_msb<2:0> and least significant bits gray_lsb<2:0> prior to being converted to corresponding thermometer coded bits th<6:0>.

In one embodiment, a finite state machine within the measurement subsystem initiates duty cycle and skew measurements, interprets the results, and adjusts the configurable clock buffer chains to minimize duty cycle and skew errors. To reduce power utilization, the measurement subsystem may operate periodically, rather than continuously. The finite state machine may perform duty cycle corrections for clock 1, duty cycle corrections for clock 2 duty cycle, and delay corrections for rising-edge to rising-edge and/or falling-edge to falling-edge delay corrections sequentially. FIG. 10 illustrates various steps of such a sequential operation. As shown, control signals ‘0’, ‘1’, ‘2’, and ‘3’ may correspond to a two-bit input to a selection circuit, e.g., a multiplexor in control logic 240. In the embodiment of FIG. 10, a selection input of ‘0’ corresponds to falling-edge to falling-edge (FE-to-FE) delay corrections between clocks Clk1 and Clk2, a selection input of ‘1’ corresponds to duty cycle correction of Clk1, a selection input of ‘2’ corresponds to a duty cycle correction of Clk2, and a selection input of ‘3’ corresponds to rising-edge to rising-edge (RE-to-RE) delay corrections between clocks Clk1 and Clk2. As shown, the system corrects duty cycle and delays between Clk1 and Clk2 in approximately 1 microsecond.

In a first startup mode, the duty cycle of each clock is rapidly optimized by simultaneously modifying both the rise time and the fall time configuration of its respective buffer chain after each measurement cycle. The skew between the two clocks is adjusted by modifying both the first and second clock rise times after each measurement cycle.

In a second operational mode, the duty cycle of each clock is adjusted non-intrusively, by incrementing modifying only the falling edge characteristics of the clock buffers after each measurement cycle. If required, the skew between the two clocks is adjusted by incrementally modifying the rise time for one clock or the other.

To minimize the number of control signals needing to be routed, the measurement subsystem outputs binary control values. A gray code is used to minimize glitching when incrementally increasing or decreasing a control value. The more-significant and less-significant portions of the control value are locally converted using Boolean logic from gray code to thermometer code to control enabling of driver elements in 350 and 330, respectively. Clocked latches synchronize changes between more-significant and less-significant portions of the control value to minimize glitching.

FIG. 8 is a flow chart describing one embodiment of a method 800 in accordance with some embodiments. As shown, method 800 includes generating edge-triggered half-rate clocks from a divider operating responsive to edges of first and second clock signals. The method further includes generating 808 a set of multi-bit control signals, each multi-bit control signal having coarse and fine components for adjusting a rising edge and a falling edge of a respective clock signal of the first and second clock signals, each multi-bit control signal incrementally updated responsive to a selection of a duty cycle correction of the respective clock signal generated 804 responsive to a comparison of a common mode signal associated with the respective clock signal to a reference voltage and delay corrections between the first and second clock signals, the delay corrections generated 806 responsive to inter-phase comparisons between the edge-triggered half-rate clocks. The coarse and fine inverter stages of a set of clock buffers generating the first and second clock signals are adjusted 810 according to the coarse and fine components of the set of multi-bit control signals, respectively.

In some embodiments, generating the edge-triggered half-rate clocks includes generating half-rate singled-ended clocks from the first and second clock signals. In such embodiments, generating the half-rate clock signals includes generating complements of the half-rate single-ended clocks using inverters, and retiming the complements and the half-rate single-ended signals according to the corresponding first and second clock signals.

In some embodiments, the edge-triggered half-rate clocks include RE-triggered half-rate clocks and FE-triggered half-rate clocks from dividers operating responsive to rising edges and falling edges, respectively, of the first and second clock signals. In such embodiments, the method includes generating delay corrections between the first and second clock signals responsive to inter-phase comparisons between the RE-triggered and FE-triggered half-rate clocks. In some embodiments, generating the edge-triggered half-rate clocks includes enabling the dividers in a predetermined order. Such a predetermined order may be implemented via a state machine or logic.

In some embodiments, the method further comprising includes synchronizing the coarse and fine components of the multi-bit control signal. In some such embodiments, synchronization includes incrementally updating the set of multi-bit control signals by latching the coarse and fine components of the multi-bit control signal according to a flag signal. In some embodiments, the set of multi-bit control signals correspond to thermometer bits. In some such embodiments, the method further includes generating the thermometer bits from a gray code.

In some embodiments, the duty cycle corrections and delay corrections are selected via a selection circuit to incrementally update the multi-bit control signals. In some embodiments, the method further includes low-pass filtering each duty cycle correction and each delay correction responsive to selection by the selection circuit. The selection circuit may include a shared low-pass filter to perform the low-pass filtering.

In some embodiments, the method includes generating the common mode signal associated with the respective clock signal by low-pass filtering the respective clock signal.

In some embodiments, generating the inter-phase comparisons includes exclusively-OR (XORing) the edge-triggered half rate clocks. In some such embodiments, the method further includes low-pass filtering the inter-phase comparisons between the edge-triggered half-rate clocks. Such low-pass filters may be local low-pass filters operating directly on the outputs of the XOR logic gate.

In some embodiments, the coarse inverter stage precedes the fine inverter stage in the sets of clock buffers. In some embodiments, each bit of the coarse component of a given set of multi-bit control signal is provided to multiple inverters connected in parallel in the coarse inverter stage. In some embodiments, each inverter stage of a given set of clock buffers comprises at least one transistor for controlling the rising edge of a given clock signal and at least one transistor for controlling the falling edge of the given clock signal. In some embodiments, at least one transistor for controlling the rising edge of the given clock signal and at least one transistor for controlling the falling edge of the given clock signal are inverted with respect to each other in the coarse and fine inverter stages.

In some embodiments, adjusting the respective coarse and fine inverter stages of the set of clock buffers includes simultaneously adjusting the rising edges and falling edges of the first and second reference signals responsive to corresponding duty cycle corrections during a start-up mode of operation.

In some embodiments, adjusting the respective coarse and fine inverter stages of the set of clock buffers includes adjusting the falling edges of the first and second reference signals responsive to corresponding duty cycle corrections during a mission mode of operation. Furthermore, adjusting the respective coarse and fine inverter stages of the set of clock buffers may include simultaneously adjusting the rising or falling edges of the first and second reference signals responsive to each delay correction during a mission mode of operation, depending on which edge was used to generate the edge-triggered half-rate clocks.

In some embodiments, comprising selecting the first and second clock signals from a main clock path or a phase-interpolator clock path. 

We claim:
 1. A method comprising: obtaining first and second clock signals having a complementary phase relationship; generating respective edge-triggered half-rate clocks from each of the first and second clock signals; generating a delay correction signal from an interphase comparison between the respective edge-triggered half-rate clocks; and adjusting, based on the delay correction signal, at least one of a rise time and a fall time of the first and second clock signals via inverter stages of a clock buffer.
 2. The method of claim 1, wherein the interphase comparison between the respective edge-triggered half-rate clocks corresponds to a logical exclusive OR (XOR) phase comparison.
 3. The method of claim 1, wherein the delay correction signal is used to update a multi-bit control signal provided to the inverter stages of the clock buffer.
 4. The method of claim 3, wherein the inverter stages comprise coarse and fine inverter stages, and wherein the multi-bit control signal comprises coarse and fine components.
 5. The method of claim 4, wherein inverters in the coarse and fine inverter stages have different sizes.
 6. The method of claim 1, further comprising generating duty cycle corrections for each of the first and second clock signals, wherein each duty cycle correction is generated by comparing a common mode signal associated with each respective clock signal to a reference voltage.
 7. The method of claim 6, further comprising adjusting the at least one of the rise time and the fall time of the first and second clock signals based on duty cycle corrections generated for the first and second clock signals, respectively.
 8. The method of claim 7, wherein adjusting the at least one of the rise time and the fall time of the first and second clock signals occurs iteratively by incrementally outputting the delay correction signal and the duty cycle corrections generated for the first and second clock signals.
 9. The method of claim 1, wherein the respective edge-triggered half rate clocks comprise one of rising-edge half rate clocks and falling-edge half rate clocks.
 10. The method of claim 1, wherein the respective edge-triggered half rate clocks comprises both rising-edge half rate clocks and falling-edge half rate clocks, and wherein the delay correction signal comprises a rising-edge-to-rising-edge component and a falling-edge-to-falling-edge component.
 11. An apparatus comprising: two frequency dividers configured to receive first and second clock signals having a complementary phase relationship, each frequency divider configured to generate a respective edge-triggered half-rate clock from a respective one of the first and second clock signals; a phase comparator configured to generate a delay correction signal from an interphase comparison between the respective edge-triggered half-rate clocks; and a clock buffer comprising inverter stages, the inverter stages configured to adjust at least one of a rise time and a fall time of the first and second clock signals based on the delay correction signal.
 12. The apparatus of claim 11, wherein the phase comparator comprises a logic exclusive OR (XOR) gate configured to generate a logical XOR phase comparison between the respective edge-triggered half-rate clocks.
 13. The apparatus of claim 11, further comprising a control signal generator configured to provide a multi-bit control signal to the inverter stages of the clock buffer, wherein the control signal generator is configured to update the multi-bit control signal using the delay correction signal.
 14. The apparatus of claim 13, wherein the inverter stages comprise coarse and fine inverter stages, and wherein the multi-bit control signal comprises coarse and fine components.
 15. The apparatus of claim 14, wherein the inverters in the coarse and fine inverter stages have different sizes.
 16. The apparatus of claim 11, further comprising a duty cycle detection circuit configured to generate duty cycle corrections for each of the first and second clock signals, wherein each duty cycle correction is generated by comparing a common mode signal associated with each respective clock signal to a reference voltage.
 17. The apparatus of claim 16, wherein the at least one of the rise time and the fall time of the first and second clock signals is further adjusted based on duty cycle corrections generated for the first and second clock signals, respectively.
 18. The apparatus of claim 17, further comprising a selection circuit configured to incrementally the delay correction signal and the duty cycle corrections generated for the first and second clock signals to iteratively adjust the at least one of the rise time and the fall time of the first and second clock signals.
 19. The apparatus of claim 11, wherein the respective edge-triggered half rate clocks comprise one of rising-edge half rate clocks and falling-edge half rate clocks.
 20. The apparatus of claim 11, wherein the respective edge-triggered half rate clocks comprises both rising-edge half rate clocks and falling-edge half rate clocks, and wherein the delay correction signal comprises a rising-edge-to-rising-edge component and a falling-edge-to-falling-edge component. 